1. Field of the Invention
This invention relates to a temperature detector for measuring high temperatures, especially suitable for use in a pressurized gas atmosphere of a high temperature about 2000.degree. C., for example, in pressurized sintering furnaces and hot isostatic presses (hereinafter referred to simply as "HIP" for brevity), and more particularly to a thermocouple temperature detector with an improved construction for supporting thermocouple wires.
2. Description of the Prior Art
Recently, large investments are made in the research and development of non-oxide ceramics such as silicon nitride (Si.sub.3 N.sub.4) and silicon carbide (SiC) which are looked upon as a high-strength material suitable for application to high-efficiency gas turbines and Diesel engines. In the manufacturing processes of Si.sub.3 N.sub.4, for example, it has been considered to use a pressurized sintering furnace which holds an N.sub.2 atmosphere of 1800.degree.-2100.degree. C. under pressure of 10-100 kgf/cm.sup.2, or a HIP apparatus which holds an N.sub.2 atmosphere of 1700.degree.-1800.degree. C. under pressure of 1000-2000 kgf/cm.sup.2.
With regard to the means for measuring temperatures in such pressurized sintering furnaces and HIP apparatus, it is desirable to employ an optical temperature detector in view of the operation in a high temperature range which in some cases exceeds 1700.degree. C. However, in the case of an optical temperature detector, it is necessary to lead the radiant light from a furnace directly to a sensor portion of the detector. For instance, in a HIP apparatus as shown in FIG. 7, openings 28 and 29 have to be formed through a pressure container 20 with upper and lower lids 21 and 22 and through a heat shielding wall 24 which is located between the inner wall surface of the pressure container and a processing chamber 23 including a support member 25 and a heater 26.
The provision of such opening 28, however, impairs the strength of the pressure container 20, while the opening 29 induces circulation therethrough of the pressurizing gas medium, exposing the inner wall surfaces of the container to a high temperature and as a result inviting large heat losses. The application of the optical temperature detector is therefore substantially difficult, and presently can be found in pressurized sintering furnaces of up to about 10 kgf/cm.sup.2 G.
As a temperature detector for pressurized sintering furnaces and HIP apparatus operating at a pressure level higher than 10 kgf/cm.sup.2 G, there are no suitable temperature detectors on the market except W-Re base thermocouples (e.g., W-Re 5/26 thermocouple, a product of HOSKINS of the United States, 0.5 mm in wire diameter). Accordingly, in the case of a HIP apparatus, attempts have been made to embed, in the heat shielding wall of the pressure container, a commercially available thermocouple which is inserted in an insulating tube and retained at the upper end of thereof, the thermocouple and insulating tube being received in a protective sheath which is closed at the fore end thereof.
However, the conventional method in which the insulating tube and thermocouple are contacted with each other in broad high temperature areas has a problem that is practically difficult to avoid shunt errors due to drops of electric insulation of the tube.
Further, the above-mentioned commercially available thermocouples normally have a small wire diameter of about 0.5 mm, so that, if applied to a HIP apparatus having a diameter of 200 mm and a length of 500 mm and a temperature range up to 2000.degree. C., breakage of the fine thermocouple wire is very likely to occur due to coarsening of crystal grains. Consequently, it is often the case that a thermocouple has a very short service life, enduring only one operation or so. This naturally hinders industrilization of the HIP apparatus of 2000.degree. C. class.